Flow type checking for Axios

Type checking with Flow is a nice addition to your code. Unfortunately, it is not so intuitive to use Flow with libraries like Axios. The main problem is lack of easily available usage examples.

Problem

Using Axios type annotations $AxiosXHR and $AxiosXHRConfig may result in error messages similar to this:

Cannot call resolve with result bound to result because string [1] is incompatible with object type [2] in type argument T [3]

or this:

Cannot call resolve with result bound to result because object type [1] is incompatible with string [2] in type argument R [3].

What are T and R and how to fix that?

Solution

If you would like to find out the answer yourself you will have to read the source code of Axios Flow implementation. The code is not so complicated if you have some experience with Flow types, but for newcomers it looks cryptic (at best).

Moving forward...

$AxiosXHR is defined as $AxiosXHR<T, R=T> which means that we have to provide at least one argument T and optionally another R.

T - input data Type, should be a type of data parameter in your Axios request

R - Response type, should define expected type for a response

As you may noticed R is optional and equals T by default which is useful especially when dealing with no request data (e.g. all GET queries) or no response data (e.g. POST query with just 200 OK as a response).

Example

As usual we have to begin with // @flow declaration, import of Axios library and appropriate type definitions:

We know that Github API endpoint /rate_limit is going to return something which should match RateLimitResponse. As we don't have any input data type there is no need to define more than one $AxiosXHR parameter (R equals T by default).

What is more, we know that async function should return a promise, thus we wrap returned type with Promise<....>.

Now we can use Axios with Flow typings.

NOTE
As mentioned by @Joey M. you may replace:

Promise<$AxiosXHR<RateLimitResponse>>

with

AxiosPromise<RateLimitResponse>

It's a hand wrapper which simplifies typings a bit. Of course it has to be imported before usage: import type { AxiosPromise } from 'axios';

Another example

Let say we have an API endpoint http://your-api-enpoint.test which will respond with string OK to every POST request with JSON data body: { getStatus: true }.

In this case Axios Flow typing for request is going to look like this: